If you drop a penny in somebody's drink which already has a penny in it, then you must down their drink, immediately.

If you attempt to drop a penny in somebody's drink, and you miss, either by ineptitude or a skilful deflection, then again, you must down the drink you were aiming for.

You may not drop a penny in a drink that you just poured for somebody. This is unsportsmanlike. You may, however, arrange for somebody else to pour the drink (without necessarily asking or informing the target).

Bring pennies ("ammunition"). Bring many of them. Look out for the guy who brings one hundred pennies to Formal and strolls up and down all the tables, individually pennying everybody's glass one at a time.

Keep a close eye on your opponents to see if other people are pennying them. You don't want to accidentally double-penny someone.

If you REALLY don't want to be pennied, you're probably best off drinking water instead of wine.

A penny is more easily visible through white wine than red wine, so you stand a better chance of being double-pennied if you drink the red.

Try not to behave too badly. Penny Game-induced drunkenness and disorder (sometimes degenerating into food fights) has resulted in Formal Hall being withdrawn completely at a number of colleges, while the Penny Game is officially banned in various other Halls. Respect the other diners.